The next Elder Scrolls V: Skyrim DLC will be called Dragonborn and will feature dragon mounts in some capacity. At least that's the word according to a post on the Bethesda forums which claims to have uncovered the information after digging through the source code of the most recently released Elder Scrolls V: Skyrim patch.

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Other key points of interest extracted from the code:

The DLC will take place on the island of Solstheim

Main locations will be Raven Rock, Miraak Temple, Castle Karstaag, and Telvanni Tower.

New Bonemold, Chitin, Nordic, and Stalhrim armour types

New creatures (not specified)

New Dwemer Artifacts (not specified)

PS3 Downloadable Content is mentioned (nothing specific)

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Obviously all this info is currently 100% unconfirmed by Bethesda, but it’s worth remembering clues to Skyrim’s Dawnguard DLC were similarly discovered in an update patch pre-release which turned out to be accurate.